1. Field of the Invention
The present invention pertains to silicone release coatings.
2. Background Art
Silicone release coatings are used in large quantities in diverse commercial fields. For example, in the structural composites field, curable resin-impregnated carbon or glass fiber prepregs are manufactured on release-coated substrates from which the tacky prepregs are separated prior to assembling into laminates and curing. Similar release substrates, generally release coated paper, are used with film adhesives, and as protective, pre-use coverings for a variety of adhesive-coated articles. Pressure sensitive labels, for example, are generally supplied on release coated substrates. It is safe to say that virtually all curable or pressure-sensitive film adhesives and articles coated with them are supplied on a release-coated substrate or backing.
Silicones have become the predominant release coating due to their superior surface qualities and favorable economics. However, because of the diverse applications in which the release coatings are used, and the varying tack of the wide variety of adhesives with which they are used, it is necessary to have the ability to tailor characteristics such as peel strength, and, in particular, the coating coefficient of friction (COF). In the past, changes in COF were rather difficult to make, often requiring the synthesis of new friction-modifying components or even wholly new release formulations. Coatings with low COF have been the most difficult in this regard.
The coatings must be capable of application to the substrate by conventional processes, such as spraying, doctor coating, printing techniques, etc. These application methods require a coating of relatively low viscosity, preferably with some degree of self-leveling characteristics. Following application to the substrate, the coating must be cured. Among the curable compositions and their associated curing mechanisms which have been used are moisture curable compositions, condensation curable compositions, addition curable compositions, and photocurable compositions. Thermocurable and photocurable addition curing systems dominate the market.
In many applications, very low levels of extractables are required. However, some silicone release coatings actually rely on the exudation of liquid organopolysiloxanes from a cured and crosslinked organopolysiloxane matrix for their release properties. Such compositions are described in European published application EP-013703. The exudation is significant enough that the latter published application is directed to employing a special substrate backcoating containing absorbent particles to absorb excess silicone oil when the release film is wound up on a roll. However, such special backcoatings do not prevent later extraction of silicon fluid upon use of the release film.
Other conventional silicone release coatings are disclosed by U.S. Pat. No. 4,184,006, incorporated herein by reference, comprising a vinyl-terminated diorganopolysiloxane, a diorganopolysiloxane having minimally three Sixe2x80x94H bonded reactive hydrogens, a platinum hydrosilylation catalyst, and a hydrosilylation inhibitor. The compositions may be applied by conventional coating and printing techniques. However, low COF coatings are difficult to obtain. For low COF coatings, solvent-based tin catalyzed condensation-curable coatings have traditionally been used. However, tin-catalyzed coatings not only contain appreciable tin residues, they have high levels of extractables as well. Moreover, most solvent based systems do not meet volatile organic compound (VOC) emission standards due to the relatively large amounts of solvents used.
It would be desirable to provide silicone coating compositions, suitable for use as release coatings, with COF values which can be adjusted over a wide range without drastic alteration of the coating composition. It would be further desirable to provide coating composition components which allow for adjustment of COF by the coating applicator rather than only the coating manufacturer. Finally, it would be desirable to provide low VOC content or VOC-free coatings which have a low level of extractables after curing. It would be yet further desirable to provide low COF coatings which do not require the use of tin catalysts.
It has now been surprisingly discovered that silicone release coatings with adjustable COF, in particular low values of COF, may be formulated as neat, solventless compositions, or as compositions containing minimal organic solvent. These release coatings may be applied by conventional coating techniques, and cure to provide release coatings having very low levels of extractables.
The present invention compositions are addition-curable or photocurable organopolysiloxane release coating compositions which contain, as a COF modifier, a very high molecular weight, and hence high viscosity ( greater than 500,000 mm2/s) organopolysiloxane which does not react substantially with the remaining ingredients of the curable organopolysiloxane composition. The release coating compositions may be defined and described in several ways, for example by weight percent composition. However, it is advantageous to consider the compositions as comprising a curable organopolysiloxane base composition, and a COF modifier, preferably supplied as a COF modifier master batch composition. By varying the amount of the latter, the COF of the cured coating composition may be varied over wide limits without necessitating fundamental changes in the base composition. Thus, release coatings with varied COF may be formulated by the applicator from a limited number of standard components.
The base composition is an addition crosslinkable system which may be thermally crosslinkable or photochemically crosslinkable. Hybrid systems are useful as well. The base compositions are conventional, and well known to those skilled in the art.
Addition crosslinkable systems contain a xe2x80x9ccrosslinkable componentxe2x80x9d which is preferably an organopolysiloxane containing hydrocarbon groups which have carbonxe2x80x94carbon unsaturation. Both ethylenically unsaturated and ethylynically unsaturated groups may be present. Preferably, the unsaturated hydrocarbon radicals are vinyl, vinylether, or xcfx89-terminal alkenyl or alkenylether groups. Allyl groups and allylether groups are also highly useful, as are also acrylates and methacrylates, maleates, fumarates, etc.
The unsaturated hydrocarbon groups may be pendant or terminal, or both pendant and terminal, and the polysiloxanes which bear these groups may be linear or branched, preferably lightly branched such that they contain less than about 5 mol percent total RSiO3/2 and SiO4/2 groups based on total mol of siloxy groups, where R is an optionally substituted hydrocarbon group. Linear, xcex1,xcfx89-divinylpolydiorganosiloxanes are preferred, more preferably xcex1,xcfx89-divinylpolydimethylsiloxanes. Thus, more generally, the unsaturated group-containing organopolysiloxanes preferably contain the following groups:
where
a and b are individually 0, 1, 2, or 3 and the sum of a+b is 3;
where c and d are individually 0, 1, or 2 and the sum of c+d is 2;
where e and f are individually 0 or 1 and the sum of e+f is 1;
where R1 is an unsaturated, optionally substituted hydrocarbon, preferably a C2-18 ethylenically unsaturated hydrocarbon, and more preferably, an xcfx89-unsaturated C2-18 hydrocarbon, optionally substituted by substituents which are non-reactive with other coating composition components. Examples of such substituents, in a non-limiting sense include, halo, especially fluoro and chloro; cyano; hydroxyl; sulfhydryl; primary, secondary, and tertiary amino, or salts thereof; alkoxy; and polyoxyalkylenyl. In the above formulae, R is an optionally substituted hydrocarbon group free of ethylenic unsaturation, or a hydroxyl or alkoxy group, preferably a phenyl or substituted phenyl group or a C1-18 alkyl group, more preferably a C1-8 alkyl group, and most preferably a methyl group. The groups R, when hydrocarbon groups, may be substituted by one or more substituents as for R1. In addition curable organopolysiloxanes, this component may be termed the xe2x80x9ccrosslinkable componentxe2x80x9d. The R alkoxy groups preferably are C1-18 alkoxy groups, more preferably C1-2 alkoxy groups. All the organosilicon compounds herein may also include such linkages as xe2x89xa1Sixe2x80x94R1xe2x80x94Sixe2x89xa1 where R1 is C1-18 alkylene, polyoxyalkylene, or similar linking groups; i.e., the organopolysiloxane backbone need not be limited to repeating Sixe2x80x94Oxe2x80x94Si linkages.
In addition to the organopolysiloxane crosslinkable components, the crosslinkable components may comprise, all or in part, a multiply unsaturated hydrocarbon, in particular, C4-18 xcex1, xcfx89-dienes, more preferably C4-8 xcex1, xcfx89-dienes. Dienes with internal multiple unsaturation as opposed to terminal unsaturation may also be used. On average, the multiply unsaturated hydrocarbon, as is the case with the unsaturated organopolysiloxanes, should contain two or more unsaturated carbonxe2x80x94carbon bonds, but may contain greater than two unsaturated bonds as well. Both the R1 substituents of the crosslinkable organopolysiloxane, the R substituents of the crosslinkable organopolysiloxane, and the multiply unsaturated hydrocarbons may contain interspersed heteroatoms, and heteroatom-containing groups such as 
and the like. Moreover, the C1-18 hydrocarbon group R, the C2-18 unsaturated hydrocarbon group R1, and the multiply unsaturated hydrocarbon may contain internal or external polyoxyalkylene ether groups, which, when external, may be terminated by hydroxyl, xe2x80x94OR, or xe2x80x94OR1 groups, where R and R1 have the before-mentioned meanings. Thus, in summary, the crosslinkable component may comprise any multiply unsaturated polyorganosiloxane or hydrocarbon moiety with suitable reactivity in hydrosilylation reactions to be able to economically form a coating.
In addition to the hydrosilylation reactions which will crosslink the thermocurable addition-crosslinkable coatings, other reactions which form crosslinking linkages may also occur, although these will not be the dominate reaction. Examples are Michael-type additions of aminoalkyl groups with unsaturated groups.
Most preferably, the crosslinkable component comprises a linear polydimethylsiloxane containing terminal or pendant unsaturated groups, and having the formula
R11RbSiOxe2x80x94(Rc1RdSiO)nxe2x80x94SiRa1
where a is 0, 1, or 2, preferably 1, and b is 1 or 2 preferably 2, and the sum of a+b is 3; where c is 0, 1, or 2, preferably 1, more preferably 0; and d is 0, 1, or 2, preferably 1 and more preferably 2; and where n is an integer from 0 to 1000 or more. The crosslinkable component is preferably an xcex1,xcfx89-divinylpolydimethyl siloxane corresponding to the formula
ViMe2SiOxe2x80x94(SiMe2O)nxe2x80x94SiMe2V1
where Me is methyl and Vi is vinyl. Preferred crosslinkable components are liquids having a viscosity of less than 100,000 cps, more preferably less than 50,000 cps, and most preferably less than 10,000 cps. An individual component may have a higher viscosity or be a solid, so long as the overall release coating composition is a liquid of sufficiently low viscosity to be applied by conventional coating and printing methods. Generally, the coating composition should be less than 10,000 cps, more preferably less than 5000 cps, and most preferably 2000 cps or lower.
The xe2x80x9ccrosslinking componentsxe2x80x9d of the addition-curable release coatings which cure by hydrosilylation, comprise an Sixe2x80x94H functional organopolysiloxane containing, on average, minimally two Sixe2x80x94H groups per molecule. Suitable Sixe2x80x94H functional crosslinking agents preferably contain at least 3 Sixe2x80x94H groups. The crosslinking organopolysiloxane is preferably comprised of the moieties:
where a, b, c, d, e, f, and R have the same meanings as hereinbefore for the crosslinkable component, and R can be similarly substituted or contain interspersed heteroatom groups. The Sixe2x80x94H functional crosslinker may also be cyclic, i.e., tetramethylcyclotetrasiloxane, hexamethylcyclotrisiloxane, pentamethyl-cyclopentasiloxane, and the like. As with the crosslinkable components, the crosslinking components are preferably linear or only lightly branched, for example containing less than 5 mol percent T (IIIxe2x80x2) and Q (IV) units. Most preferably, the Sixe2x80x94H functional crosslinker bears from 0.05 to about 1.6 weight percent Si-bound hydrogen, which is located along the polymer backbone as opposed to merely the crosslinker termini. More preferably, the amount of Si-bound hydrogen is between 0.2 and 1.6 weight percent, more preferably between 1.0 and 1.46 weight percent. The crosslinker is preferably a liquid, but may be a solid so long as the overall release coating composition is liquid, as described previously. The viscosity of liquid, Sixe2x80x94H functional crosslinkers is preferably from 2 to 10,000 cps, more preferably 4 to 1000 cps, and most preferably 4 to 130 cps.
In addition or in lieu of the crosslinkable components (I) to (IV) and crosslinking components (Ixe2x80x2) to (IIIxe2x80x2) and (IV), the compositions may comprise organopolysiloxanes containing a combination of moieties (I) to (IV) and (Ixe2x80x2) to (IIIxe2x80x2). In other words, all or a portion of the release coating may comprise organopolysiloxanes containing both unsaturated hydrocarbon groups as well as silicon-bonded hydrogen in the same molecule. Such components are not preferred, however.
The hydrosilylation-curable addition curable release coatings necessarily contain a hydrosilylation catalyst. These catalysts are by now well known, and may comprise a number of transition metals and transition metal compounds, particularly those of the precious metals, more particularly compounds of platinum, palladium, or rhodium, and most preferably platinum and platinum compounds. Preferably, vinylsiloxane platinum compounds such as those described in U.S. Pat. Nos. 3,419,593, and 3,715,334 (xe2x80x9cKarstedtxe2x80x9d catalysts) are utilized. The amount of hydrosilylation catalyst is preferably from 1 to 100 parts by weight per million parts by weight of the sum of crosslinkable components, crosslinker components, and any reactive diluents, as hereinafter described, preferably 5 to 60 parts per million.
The addition-curable release compositions preferably contain a hydrosilylation retarder, or xe2x80x9cinhibitorxe2x80x9d as well. Numerous inhibitors are known, including a variety of sulfur-containing compounds. Preferred retarders are yneols such as 2-methyl-3-butyne-3-ol and ethynylcyclohexanol. Other suitable retarders are disclosed in U.S. Pat. No. 4,184,006, which is incorporated herein by reference.
The addition-curable composition may contain reactive diluents such as mono-unsaturated, low molecular weight hydrocarbons, for example, 1-hexene, 1-octene, and the like. In general the amount of Sixe2x80x94H crosslinker is adjusted so as to enable complete reaction of such reactive diluents. Otherwise, the amount of extractables may increase, as will also the amount of VOC liberated during the cure cycle. Cycloolefins such as cyclohexene, cyclooctene, and the like are also suitable reactive diluents, as are styrene, xcex1-methylstyrene, p-methylstyrene, allylalcohol, and the like. Low molecular weight and thus low viscosity, 0.1 cps to 150 cps siloxanes and organopolysiloxane oligomers containing but a single unsaturated hydrocarbon moiety may be used as reactive diluents as well. Low molecular weight reactive diluents are generally added to lower the viscosity of the release coating composition without the necessity of including fugitive solvents.
Thermocurable compositions containing organopolysiloxanes having siloxy moieties I to IV, and optionally hydrocarbon di- or polyenes may be formulated to contain catalysts such as hydroperoxy, peroxy, peroxyketone, azo compounds and the like as free radical polymerization initiators. Such catalysts catalyze the addition reaction between unsaturated hydrocarbons, and are also thermocurable compositions. In such compositions, a polyunsaturated component must be present to provide a sufficient degree of crosslinking.
The base compositions of the present invention may also be photocurable. Such compositions contain unsaturated compounds as the reactive ingredients, i.e., the xe2x80x9ccrosslinkablexe2x80x9d unsaturated hydrocarbon group-containing compounds described previously, with the exception that a multiply unsaturated hydrocarbon cannot be used alone; at least a portion of the composition must contain an organopolysiloxane bearing two or more unsaturated hydrocarbon groups, preferably an organopolysiloxane bearing at least two ethylenically unsaturated hydrocarbon groups. However, when multiply unsaturated hydrocarbons are utilized, organopolysiloxanes containing but a single unsaturated group may be used as well. As with free radical polymerizable, thermocurable compositions, the number of polymerizable unsaturated groups must be sufficient to provide the requisite degree of crosslinking.
The photocurable compositions preferably contain no Sixe2x80x94H functional organopolysiloxanes, hydrosilylation catalyst, or hydrosilylation retarder. However, a photocatalyst is required. Suitable photocatalysts include a wide variety of catalysts, including iodonium and sulfonium compounds. Numerous suitable photocatalysts are available from the Ciba Geigy as Irgacure(trademark) catalysts.
The release coating may contain any additives normally employed in release coatings, for example viscosifiers, thixotropes, fillers, plasticizers, dyes, pigments, biocides, antioxidants, UV stabilizers, etc. The coating may also contain conventional friction-modifiers. Resinous organopolysiloxanes, in particular MT and MQ resins, both non-functional, unsaturated hydrocarbon-functional, or Sixe2x80x94H functional, may be added as well. In this sense, with respect to these resin components, xe2x80x9cnon-functionalxe2x80x9d means unreactive in hydrosilylation-curable compositions, i.e., having no reactive functional groups, or having a functionality other than unsaturated hydrocarbon or Sixe2x80x94H.
Solvents are preferably excluded from the curable release coating compositions. The term xe2x80x9csolventsxe2x80x9d does not include reactive diluents. The latter react into the formulation, while solvents are fugitive. When solvents are employed, they constitute less than 20 weight percent of the release coating formulation, preferably less than 10% by weight, and more preferably less than 5% by weight. No solvent, or a solvent content of less than 1 or 2% by weight are most preferred. If solvents are used, solvents which meet the United States EPA requirements for xe2x80x9cnon-VOCxe2x80x9d solvents are preferred.
The compositions heretofore described are termed silicone release coating xe2x80x9cbase compositionsxe2x80x9d, as they contain the necessary ingredients to lay down and subsequently cure to form a silicone coating layer. Thus, a xe2x80x9cbase compositionxe2x80x9d is a curable organopolysiloxane composition capable of curing to a solid film, and a xe2x80x9cbase composition componentxe2x80x9d is a component of such a composition.
Whichever base composition is utilized, the inventive compositions also contain preferably from about 0.5 weight percent to about 20 weight percent, more preferably 1 to 10 weight percent, yet preferably 0.75 to 7 weight percent, and still more preferably 1.0 weight percent to about 6 weight percent of a very high molecular weight organopolysiloxane which is substantially non-functional. The viscosity of the very high molecular weight organopolysiloxane is preferably in excess of 650,000 mm2/s, more preferably in excess of 750,000 mm2/s, yet more preferably in excess of 850,000 mm2/s, and most preferably 1,000,000 mm2/s or higher.
The upper molecular weight limit of the COF modifier or xe2x80x9cadditivexe2x80x9d is limited by the increasing viscosity of the release coating at the level of additive utilized; the coating must be coatable or printable using conventional techniques; the lower limit is based on the level of extractables, which is preferably less than 5 % by weight over the amount of extractables of the base composition alone, preferably 3 % by weight or less over the amount of extractables of the base composition, and most preferably not more than 2% over the level of extractables of the base composition. In the lower viscosity range, the amount of extractables tends to be high. Thus, at 500,000 mm2/s or below, while COF is excellent, extractables are high. 400,000 mm2/s represents a practical lower limit of viscosity of the COF additive, unless modified by a limited number of functional groups or other groups which decrease the mobility of the additive and hence its ability to be exuded or extracted.
As indicated in the immediately preceding paragraph, the amount of extractables can be lowered somewhat by providing partial functionality to the additive molecules, i.e., by providing alkenyl functionality (for addition or photocurable systems) or Sixe2x80x94H functionality (for hydrolyzation-addition curable systems) or another functionality which is reactive with the base composition components, i.e., amino functionality, reactive via Michael type-reactions with the crosslinkable component. However, if functionality is provided, it is desirably less than 1.0 functional groups, on average, per additive molecule, more preferably less than 0.5 functional groups, on average, per molecule, otherwise the COF lowering ability will be lost.
The COF polyorganosiloxanes additives preferably contain no reactive functional groups, reactive meaning reactive with one or more of the components of the base composition. As stated previously, a minor amount of reactive functional groups may be tolerated. However, if an excess of functional groups are present, the additive may react with the base composition to such an extent that it becomes part of the cured resin itself, presumably distributed evenly within the matrix of the cured coating. The additive will thus be substantially unavailable at the surface of the coating, where its friction-lowering capabilities are desired. While increasing the amount of additive would increase the amount available at the surface of the coating, the increase in viscosity may not be tolerable in the coating process. However, it would not depart from the spirit of the invention to include a minimal degree of functionality which would not be sufficient to completely immobilize the additive within the coating polymer. Such an amount is, on average, less than 1 reactive functional groups per molecule, preferably less than 0.5 groups per molecule, and most preferably less than 0.1 groups, on average, per molecule. Again, the term xe2x80x9creactivexe2x80x9d means reactive with the base composition or a component thereof.
Preferably, higher molecular weight, xe2x80x9cnon-functionalxe2x80x9d (essentially unreactive with the base composition components under the storage and/or curing conditions) higher molecular weight organopolysiloxanes, i.e., those with a viscosity (25xc2x0 C.) in excess of 800,000 mm2/s, preferably about or in excess of 1,000,000 mm2/s, and most preferably in the range of 850,000 mm2/s to 1,500,000 mm2/s are used. COF additives having a viscosity in excess of ca. 2,000,000 mm2/s may be particularly valuable with base compositions of exceptionally low viscosity. Otherwise, the coating composition viscosity may be increased to the extent that coating is difficult. However, there is no specific upper limit to the viscosity of the COF-lowering additive.
Preferred additives are organopolysiloxanes having the formula
R3SiOxe2x80x94(SiR2O)nxe2x80x94SiR3
where R has the meaning given earlier, but is preferably the methyl group and n is such to provide the desired molecular weight and viscosity. The additives may also be lightly crosslinked, for example containing up to 5 mol percent of RSiO3/2 and SiO4/2 groups based on the total moles of siloxy groups. Substitution of long chain alkyl groups, halogenated, particularly prefluorinated alkyl groups, and phenyl groups for the preferred methyl groups can be utilized to alter the surface characteristics. The various R groups may also be substituted, to a minor extent, i.e., less than 10 mol percent, by non-reactive functionality such as aminoalkyl, hydroxyl, halo, cyano, etc. Other non-reactive groups may be present as well.
Most preferably, the COF additive is not added to the release base composition directly during manufacture, but is formulated as a relatively lower viscosity master batch which may then be added advantageously either during manufacture or by the coating applicator. In general, it is not economical for the applicator to mix the very high molecular weight additive directly into a low viscosity base formulation or a component thereof, as special mixing equipment is required. While such equipment is commonly available to manufacturers of polysiloxanes, it is not commonly available to end users without additional capital expenditure. The COF master batch additive, to be readily blendable without resort specialized mixing apparatus should have a viscosity less than 30,000 cps, preferably less than 25,000 cps, and more preferably 20,000 cps or lower. A range of 12,000-15,000 cps has been found suitable, for example.
Most preferably, the master batch contains high molecular weight additive dissolved or dispersed in further base composition or a component thereof. For example, an addition curable composition containing a crosslinkable component and a crosslinker component, but not necessarily containing either catalyst or inhibitor, may be used. A fully catalyzed/inhibited base composition is also satisfactory. Such master batches may contain from less than 10 weight percent to more than 60 weight percent of additive, preferably 15 weight percent to 40 weight percent, and most preferably 20 weight percent to 35 weight percent. The maximum concentration of additive will be dependent upon the viscosity of the additive and the viscosity of the non-additive component. The additives may be liquid, wax-like, semi-solid, or solid. The nature of the additive is generally dictated by its structure and molecular weight. Higher molecular weight additives will, in general, be of higher viscosity, and yield higher corresponding master batch viscosity. For a trimethylsilyl-terminated polydimethylsiloxane of 1,000,000 mm2/s viscosity, a concentration of 25 weight percent additive in a conventional base composition has proven satisfactory as a master batch.
In lieu of a base composition as the non-additive medium of the master batch, a reactive diluent may be used. For example, 1-hexene, styrene, methylacrylate, etc., may be used as a reactive diluent. By the term xe2x80x9creactive diluentxe2x80x9d is meant a diluent which will react with the base composition and be chemically bound therein. For example, in addition curable systems, reactive diluents may be low molecular weight, low viscosity fluids with ethylenic unsaturation. Reactive diluents may also be hydrido-functional, although these are less preferred. Examples are tetramethyldisiloxane, pentamethyldisiloxane, tetramethylcyclotetrasiloxane, etc. For systems which are thermocurable by free radical addition and for photocurable systems reactive diluents may also be ethylynically unsaturated moieties such as those previously identified.
Solvents, particularly conventional solvents should preferably be avoided, as otherwise, the ability to supply compositions meeting low VOC requirements may be compromised. However, it would not depart from the spirit of the invention to add a most minor amount of organic solvent, preferably less than 20 weight percent, more preferably less than 10 weight percent, and most preferably 5% or less. Less than 2% solvent is considered xe2x80x9csubstantially solvent free.xe2x80x9d
If solvents are used, they are preferably solvents which exhibit little or no tendency to photochemically generate ozone and/or smog, in particular solvents which exhibit an amount of photochemical ozone or smog generation which is less than that generated by ethane. These solvents may be termed xe2x80x9cnon-VOCxe2x80x9d solvents, and some are disclosed in U.S. Pat. No. 6,048,471. In general, solvents, whether traditional organic solvents, low-VOC solvents, or non-VOC solvents, are added to promote rapid drying, improve leveling or substrate adhesion, and in particular, to lower coating composition viscosity.
When master batches of COF additive are employed, they may be added to a conventional coating composition preferably in amounts of from 1 weight percent to 60 weight percent based on the weight of the total composition, more preferably 2% to 35%, and most preferably 10% to 30%. The actual amount will vary with the % solids of the master batch, i.e., the weight percent of COF additive. In lieu of including the entire COF additive in the xe2x80x9cmaster batch,xe2x80x9d a portion of the additive may be included in the base composition or a component thereof, and the remainder supplied as a xe2x80x9cmaster batch.xe2x80x9d The amounts useful will be the amount required to supply the requisite weight percentage of COF additive as previously disclosed.
Thus, in one embodiment, the subject invention pertains to a silicone coating composition having a low coefficient of friction (COF), and containing from 0.5 to 20 weight percent of a COF reducing additive comprising a linear or lightly branched organopolysiloxane having a viscosity in excess of 400,000 mm2/s; and as a curable silicone base composition, one or more of b)i) a thermocurable, addition-crosslinkable base composition comprising
b)i)1)one or both of a crosslinkable, unsaturated hydrocarbon group-containing organopolysiloxane or multiply unsaturated hydrocarbon compound; b)i)2) an Sixe2x80x94H functional organopolysiloxane crosslinker; and b)i)3) a hydrosilylation catalyst;
b)ii) a thermocurable, addition crosslinkable base composition comprising one or more unsaturated hydrocarbon-functional organopolysiloxanes and a thermally activated catalyst effective to catalyze the addition reaction between said unsaturated hydrocarbon groups; and/or
b)iii) a photocurable, addition crosslinkable base composition comprising one or more unsaturated hydrocarbon-functional organopolysiloxanes and a photo-activated catalyst effective to catalyze the addition reaction between said unsaturated hydrocarbon groups; wherein said COF reducing additive preferably contains on average less than 1 reactive functional group per molecule which react with an Sixe2x80x94H functional organopolysiloxane or an unsaturated hydrocarbon group under the cure conditions of the silicone coating composition.
Further, in a second embodiment, the invention pertains to a master batch suitable for addition to a silicone release coating base composition to change the coefficient of friction (COF) thereof, this master batch comprising from 5 weight percent to 60 weight percent of one or more linear or lightly branched organopolysiloxanes having a viscosity in excess of 400,000 mm2/s, dissolved or dispersed in a low viscosity release coating base composition, a low viscosity release coating base composition component, or a reactive diluent, wherein the master batch contains less than 20 weight percent of organic solvent, and has a viscosity preferably less than 50,000 cps, preferably less than 30,000 cps.
In an additional embodiment, the invention pertains to a method of altering the coefficient of friction (COF) of a curable silicone release coating, this method comprising adding to the curable release coating prior to coating upon a release substrate, an effective amount of a high molecular weight organopolysiloxane having a viscosity in excess of 400,000 mm2/s, in an amount of from about 0.5 to about 10 percent by weight relative to the total coating composition weight.
A further embodiment pertains to the a process for the manufacture of a low coefficient of friction release coating, this process comprising a) selecting a curable silicone release composition; b) preparing a low COF master batch additive of low viscosity by dissolving a high molecular weight polyorganopolysiloxane with a viscosity in excess of 400,000 mm2/s in a low viscosity liquid, the master batch having a viscosity of less than 50,000 cps; and c) mixing the master batch b) into the curable silicone release composition.
While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.